On the Motion of the Heart and Blood in Animals, by William Harvey

Chapter XII

That There is a Circulation of the Blood is Shown from the Second Position Demonstrated

If these things be so, another point which I have already referred to, viz., the continual passage of the blood
through the heart will also be confirmed. We have seen, that the blood passes from the arteries into the veins, not
from the veins into the arteries; we have seen, farther, that almost the whole of the blood may be withdrawn from a
puncture made in one of the cutaneous veins of the arm if a bandage properly applied be used; we have seen, still
farther, that the blood flows so freely and rapidly that not only is the whole quantity which was contained in the arm
beyond the ligature, and before the puncture was made, discharged, but the whole which is contained in the body, both
that of the arteries and that of the veins.

Whence we must admit, first, that the blood is sent along with an impulse, and that it is urged with force below the
ligature; for it escapes with force, which force it receives from the pulse and power of the heart; for the force and
motion of the blood are derived from the heart alone. Second, that the afflux proceeds from the heart, and through the
heart by a course from the great veins; for it gets into the parts below the ligature through the arteries, not through
the veins; and the arteries nowhere receive blood from the veins, nowhere receive blood save and except from the left
ventricle of the heart. Nor could so large a quantity of blood be drawn from one vein (a ligature having been duly
applied), nor with such impetuousity, such readiness, such celerity, unless through the medium of the impelling power
of the heart.

But if all things be as they are now represented, we shall feel ourselves at liberty to calculate the quantity of
the blood, and to reason on its circular motion. Should anyone, for instance, performing phlebotomy, suffer the blood
to flow in the manner it usually does, with force and freely, for some half hour or so, no question but that the
greatest part of the blood being abstracted, faintings and syncopes would ensue, and that not only would the arteries
but the great veins also be nearly emptied of their contents. It is only consonant with reason to conclude that in the
course of the half hour hinted at, so much as has escaped has also passed from the great veins through the heart into
the aorta. And further, if we calculate how many ounces flow through one arm, or how many pass in twenty or thirty
pulsations under the medium ligature, we shall have some grounds for estimating how much passes through the other arm
in the same space of time: how much through both lower extremities, how much through the neck on either side, and
through all the other arteries and veins of the body, all of which have been supplied with fresh blood, and as this
blood must have passed through the lungs and ventricles of the heart, and must have come from the great veins, we shall
perceive that a circulation is absolutely necessary, seeing that the quantities hinted at cannot be supplied
immediately from the ingesta, and are vastly more than can be requisite for the mere nutrition of the parts.

It is still further to be observed, that in practising phlebotomy the truths contended for are sometimes confirmed
in another way; for having tied up the arm properly, and made the puncture duly, still, if from alarm or any other
causes, a state of faintness supervenes, in which the heart always pulsates more languidly, the blood does not flow
freely, but distils by drops only. The reason is, that with a somewhat greater than usual resistance offered to the
transit of the blood by the bandage, coupled with the weaker action of the heart, and its diminished impelling power,
the stream cannot make its way under the ligature; and farther, owing to the weak and languishing state of the heart,
the blood is not transferred in such quantity as wont from the veins to the arteries through the sinuses of that organ.
So also, and for the same reasons, are the menstrual fluxes of women, and indeed hemorrhages of every kind, controlled.
And now, a contrary state of things occurring, the patient getting rid of his fear and recovering his courage, the
pulse strength is increased, the arteries begin again to beat with greater force, and to drive the blood even into the
part that is bound; so that the blood now springs from the puncture in the vein, and flows in continuous stream.